There are only 15 spots available on this life-saving medical trial. And this Sydney family need two of them.

The Donnells had happy, “normal” children. And then the unthinkable happened. Twice.

From the outside, Megan and Al Donnell looked like they had it all. This family of four from Freshwater in northern Sydney have two beautiful children – Isla, 5 and Jude, 3 – and were enjoying life. That is, until they started to notice that their daughter wasn’t speaking at the same rate as other children.

They started doing special activities to help her development but results were limited. Then, the call no parent wants to face. Her new school said they had serious concerns and asked her worried parents to have her properly assessed.

Isla was diagnosed with Sanfilippo Syndrome, a rare genetic disease that affects neurological function. For children with Sanfilippo, they are born seeming completely normal but at around the age of two or three, they start to regress.

The deadly metabolic disorder is officially classified as ultra-rare, affecting one in 70,000 births.

"The doctors, I can remember early on, just kept saying to us, “Oh, but this is so rare. But this is so rare.”' Megan told 60 Minutes tonight. "But that doesn’t matter. It doesn’t matter how rare it is to us, because it happened to us."

And then it happened again. 

Due to Isla's diagnosis, these shocked parents were urged to have their son tested, 'just in case'. There was a 1 in 4 chance he too would have the syndrome. He does. Now Megan and Al face life caring for two very sick children.

"It was almost harder, Jude’s diagnosis, because then we had nothing," Megan told 60. "Whereas when we thought that Jude was OK, or was going to be OK, we knew that he would be the hope that kept us going. And so I think when we got the diagnosis for him, it was… It just finished us, didn’t it?

"The English language does not contain words that adequately describe the impact that this double diagnosis has had on our family. We have been absolutely shattered and devastated and this is beyond anything we ever imagined possible."

Isla and Jude are currently in stage one of of the disease which is that they are lagging behind other kids their age. Next they will experience behavioural difficulties including regular periods of hyperactivity and the urge to chew on things. They will be at risk of hurting themselves due to neurological deterioration and they will lose the ability to use the toilet. The next progression is that these kids will begin to slow down. They won't be able to move without assistance. Then, they'll lose the ability to eat and swallow. A feeding tube will be inserted.

Isla and Jude will be lucky to reach their teenage years.

"It is a terrible, terrible tragedy, but we feel there is hope," Megan told the Daily Telegraph. "There have been quite incredible breakthroughs in the gene therapy field in recent years. "

A miracle cure, previously tested only on mice, is going to start human trials in the American state of Ohio. After a herculean effort by themselves and other Sanfilippo parents around the world to raise money to stage the trial, the Donnells now have to hope against hope that not just one, but both of their children might get accepted onto the program. But the program can only take 15 children.

"And it’s horrible to say - I mean, I sound like that’s a competition but it’s the reality and there are lots of parents all around the world just like us with the same hope," says Megan of the stress of trying to get their children a place in the trial. "Only 15 places worldwide. So we’re realistic that it’s unlikely that we’ll get two of those places."

Their major hope is that the clinical trial is such a huge success that the cure is offered to all sufferers and they manage to save both of their children before too much damage is done.

To support families like the Donnells, please visit this website which is helping to fund the clinical trial. Click here.

These children have no idea what they are facing. So far, they are yet to experience the true debilitation of their disease:

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